A Sedna-like body with a perihelion of 80 astronomical units
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A Sedna-like body with a perihelion of 80 astronomical units

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  • 1. LETTER doi:10.1038/nature13156 A Sedna-like body with a perihelion of 80 astronomical units Chadwick A. Trujillo1 * & Scott S. Sheppard2 * The observable Solar System can be divided into three distinct regions: the rocky terrestrial planets including the asteroids at 0.39 to 4.2 astronomical units (AU) from the Sun (where 1 AU is the mean distance between Earth and the Sun), the gas giant planets at 5 to 30AU from the Sun, and the icy Kuiper belt objects at 30 to 50AU fromthe Sun.The 1,000-kilometre-diameterdwarf planetSedna was discovered ten years ago and was unique in that its closest approach to the Sun (perihelion) is 76 AU, far greater than that of any other Solar System body1 . Formation models indicate that Sedna could be a link between the Kuiper belt objects and the hypothesized outer Oort cloud at around 10,000AU from the Sun2–6 . Here we report the presenceof a second Sedna-likeobject, 2012VP113, whose perihelion is 80 AU. The detection of 2012VP113 confirms that Sedna is not an isolated object; instead, both bodies may be members of the inner Oort cloud, whose objects could outnumber all other dynamically stable populations in the Solar System. The inner Oort cloud objects probably formed on nearly circular orbits, allowing them to accumulate mass efficiently6–9 , and were later perturbed into the eccentric orbits we see today. We define an inner Oort cloud object as a body whose orbit is not readily formed with the known mass in the Solar System. This typically means a perihelion greater than 50 AU (beyond the range of significant Neptune inter- action) and a semi-major axis in the range 150 AU , a , 1,500 AU. At above 1,500 AU objects maybeconsideredto bein the outerOort cloud, as galactic tides start to become important in the formation process10 . Sedna and 2012 VP113 are the clearest examples of inner Oort cloud objects because they donotinteract significantly with any of theknown planets owing to their extremely distant perihelia (Fig. 1 and Table 1). There are two main models for inner Oort cloud object formation. In the first model, planet-sized object(s) in the outer Solar System may perturb objects from the Kuiper belt outward to inner-Oort-cloud orbits. These planet-sized objects could either remain (unseen) in the Solar System5,11 or have been ejected from the Solar System during the creation of the inner Oort cloud12 . In the second model, close stellar encounter(s) can create the inner Oort cloud objects, possibly within the first ten million years (Myr) of the Sun’s life, when it resided within its birth cluster1,3,4,11,13–17 . The outer Oort cloud objects could be created from galactic tides18 , but the more tightly bound inner Oort cloud objectsaredifficulttocreate without aclosestellarencounter19 .A third, less explored, model is that the inner Oort cloud objects are captured extrasolar planetesimals lost from other stars that were in the Sun’s birth cluster2,3,20 . Each theory of inner Oort cloud object formation pre- dicts different orbital configurations for the population. Therefore, as more inner Oort cloud objects are discovered, their orbits will provide strong constraints on the inner Oort cloud object formation models and thus our Solar System’s evolution. To place constraints on the inner Oort cloud object population, we constructed a simple simulation of the observational biases affecting our survey(Methods). Wefindthree basic results fromthisanalysis: (1) there appears to be a paucity of inner Oort cloud objects with perihelia in the range 50–75 AU, suggesting that the inner Oort cloud object popu- lation could increase with q . 75 AU; (2) the existence of 2012 VP113 means that the inner Oort cloud object population must reach down to fairly small semi-major axes of about 250 AU; and (3) there are no obser- vational biases that can explain the clustering of the argument of peri- helion (v) near 340u for inner Oort cloud objects and all objects with semi-majoraxesgreaterthan150 AU andperiheliagreaterthanNeptune. Although our survey was sensitive to objects from 50 AU to beyond 300 AU, no objects were found with perihelion distances between 50 AU and 75 AU, where objects are brightest and easiest to detect. This was true for the original survey that found Sedna1 and the deeper follow-up survey21 . Figure 2 illustrates this—most simulated objects (and the real objectsSednaand2012 VP113)arefoundneartheinneredgeofthepopu- lation.IftheinnerOortcloudobjectshadaminimumperihelionof50AU and followed a size distribution like that of the large end of all known *These authors contributed equally to this work. 1 Gemini Observatory, 670 North A‘ohoku Place, Hilo, Hawaii 96720, USA. 2 Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015, USA. Sedna Perihelion distance (AU) 4020 0.0 0.2 0.4 0.6 0.8 1.0 8060 Eccentricity 2012 VP113 Figure 1 | Sedna and 2012 VP113 are clear dynamical outliers in the Solar System. Eccentricity versus perihelion distance for the approximately 1,000 minor planets with well-determined (multi-year) orbits beyond 10 AU are depicted. There are no known objects with closest-approach distances (perihelia, q) between 55 AU and 75 AU, even though such objects would be closer, and should therefore be brighter and easier to detect than Sedna or 2012 VP113. This suggests there may be a paucity of inner Oort cloud objects with q , 75 AU. The perihelia of Sedna and 2012 VP113 are much too distant from Neptune (30 AU) for their existence to be explained by the known mass in the Solar System. All error bars are smaller than the data symbols. 2 7 M A R C H 2 0 1 4 | V O L 5 0 7 | N A T U R E | 4 7 1 Macmillan Publishers Limited. All rights reserved©2014
  • 2. small-body reservoir distributions22,23 (q9 5 5), there would be only a 1% chance of finding 2012VP113 and Sedna with perihelion greater than 75 AU and no objects with perihelion less than 75 AU. Therefore, we conclude that there are few (although probably not zero) inner Oort cloud objects in the 50–75 AU region. Some stellar encounter models that include the capture of extrasolar material predict a strong inner edge to the perihelion distribution of objects, which is consistent with our observations2 . Our simulation suggests that the inner Oort cloud contains about 1/80th of an Earth mass of material for our favoured q9 5 5 model, al- though this can vary based on the assumed size distribution (Methods and Extended Data Table 1). This is similar to that of the Kuiper belt objects(KBOs),whichhaveabout1/100thofanEarthmassofmaterial22 . We estimate that about 430z400 {240 1sð Þ inner Oort cloud objects brighter than r 5 24.3 remain undiscovered throughout the sky. In addition, 900z800 {500 1sð Þ bodies are expected with diameter larger than 1,000 km (most of which are extremely distant), which is statistically consistent with detailed model estimates based only on the discovery of Sedna21 . Our simulations assume that the inner Oort cloud objects are longitu- dinally symmetric, but it is interesting that both 2012 VP113 and Sedna were found at similar sky locations. If further inner Oort cloud object discoveries prove this effect to be statistically significant, this would strongly constrain inner Oort cloud object formation models. Both Sedna and 2012 VP113 have similar arguments of perihelion (v 5 311u and 293u, respectively). The orbital element v describes the angle between the point of perihelion and where a body’s orbit crosses thecelestialplane.Surprisingly,thisvsimilarity issharedforallknown objects with semi-major axes greater than 150 AU and perihelia greater than that of Neptune (v < 340u 6 55u; see Fig. 3, Extended Data Fig. 1 and Extended Data Table 2). These extreme scattered disk objects may have a similar origin to Sedna or 2012VP113 but their much lower peri- helia (q ,49AU) allow their existence to be explained through resonant interactions with Neptune, except for 2004VN112, 2010GB174 and pos- sibly2000 CR105 (refs10,24–27).Wefindthatthisvclusteringcannotbe due to observational bias for two reasons: (1) any bias for v 50u is the same as the bias for v 5 180u; and (2) the surveys that have found the objects with clustered v were often off-eclipticorall-sky surveys, which wouldnothaveabiasforeitherv 50uor180u(Methods).Thus,wecon- clude that the v clustering is a real effect. The Lidov–Kozai effect is the best known dynamical mechanism for constraining the v of a minor planet28 . This three-body interaction can create outer Oort cloud objects with v preferentially near 0u and 180u early in the history of the Solar System when the Sun was still among its nascent cohort of perturbing stars29 . This cannot explain the v < 0u trend today, because v circulates owing to the presence of the giant planets. By numerically simulating the effect of the known mass in the Solar System on the inner Oort cloud objects, we confirmed that inner Oort cloudobjectsshouldhaverandomv (Methods).Thissuggeststhat a massive outer Solar System perturber may exist and restricts v for the inner Oort cloud objects. We numerically simulated the effect of a super-Earth-mass body at250 AU andfoundthat v for inner Oortcloud objects librated around 0u6 60u for billions of years (see Extended Data Figs 2 and 3). This configuration is not unique and there are many possibilities for such an unseen perturber. A super-Earth-mass body at 250 AU with very low albedo would be fainter than current all-sky survey detection limits, as would larger and more distant perturbers30 . Semimajor axis (AU) Perihelion(AU) 200 200 180 160 80 60 140 120 100 Perihelion(AU) 200 180 160 80 60 140 120 100 300 400 500 600 700 800 Semimajor axis (AU) a b 200 300 400 500 600 700 800 Figure 2 | Results from the observational bias simulation. The simulated inner Oort cloud objects (dots) are shown, along with the simulated detections in our survey (blue crosses) and the most sensitive all-sky survey so far, which re-detected Sedna21 (red crosses) for two different inner Oort cloud object models: our favoured model (a) and a comparison model (b). We have displayed ten simulated survey realizations per survey. The true values for Sedna and 2012 VP113 are shown as red and blue circles, respectively, both with error bars much smaller than theplot symbol (there are no error bars associated with all other symbols, which describe simulated data). There is one difference between the two models, the minimum perihelion for the underlying population (75 AU in a versus 50 AU in b). Our observational results favour the model in a because both Sedna and 2012 VP113 were found within a few AU of their perihelion. This indicates that the inner Oort cloud objects may have increasing numbers with increasing distance. Table 1 | The orbital elements of the inner Oort cloud objects Object q (AU) a (AU) e i (deg) V (deg) v (deg) tp (JD) Epoch (JD) 2012 VP113 80 266 0.698 24.051 90.806 293 2,443,300 2,455,857.1 Sedna 76.23 542 0.8593 11.92840 144.466 311.01 2,479,197 2,456,400.5 The barycentric elements for 2012 VP113 and the heliocentric elements for Sedna (from the NASA Jet Propulsion Laboratory Horizons tool for dwarf planets http://ssd.jpl.nasa.gov/?horizons) are given as semi- major axis (a), eccentricity (e), inclination (i), longitude of ascending node (V), argument of perihelion (v), and perihelion distance (q), which has been derived from a and e. Time of perihelion (tp) and epoch are specified in terms of the Julian date (JD). The precision to which quantities are known is reflected in the number of significant digits. We define inner Oort cloud objects as having q . 50 AU and 150 AU , a , 1,500 AU (see main text). RESEARCH LETTER 4 7 2 | N A T U R E | V O L 5 0 7 | 2 7 M A R C H 2 0 1 4 Macmillan Publishers Limited. All rights reserved©2014
  • 3. Although sucha perturber could keep the v near 0u, howobjects ini- tially obtained v < 0u and not 180u isunknown.Stochastic events,such as a single strong stellar encounter or rogue planet could produce such asymmetric populations. A strong stochastic encounter could also pro- duce the low semi-major axis of 2012VP113. It is possible that the v clustering indicates that all q . 30 AU and a . 150 AU bodies formed by a similar process, although bodies on elliptical orbits with q less than approximately 45AU can beexplained byNeptune interactions. Models ofgalactictideshavedifficultyproducinginnerOortcloudobjectshaving low semi-major axes (like Sedna) in the absence of stellar encounters19 so theeven lowersemi-majoraxisof 2012 VP113 makestides extremely unlikely. If the inner Oort cloud objects were created from the grav- itational tide within our Sun’s birth cluster, the low semi-major axis of 2012 VP113 constrains the birth cluster to have been a moderate10,000 solar masses per cubic parsec or more, depending on the cluster model assumed15,29 .The moderately red opticalcolours observedfor2012 VP113 (Methods and Extended Data Table 3) are consistent with formation in the gas giant region and not in the ultra-red dominated classical Kuiper belt. Assuming a moderate albedo, 2012 VP113 is 450 km in diameter,largeenoughtobeconsidered adwarfplanetifitiscomposed largely of ice. METHODS SUMMARY For this work, we conducted two main observational investigations. First, we sur- veyed 52 square degrees of sky for new inner Oort cloud objects using the Dark Energy Camera (DECam) attheCerroTololoInter-AmericanObservatory(CTIO) 4-m telescope. Each patch of sky was imaged three times with 1.5 h to 2 h between images, giving us the ability to detect the motion of objects beyond 300AU when compared to background stars. Analysis software aided our search for moving objects. After its discovery at the CTIO 4-m telescope, we measured the position and colour of 2012VP113 using the Magellan 6.5-m telescope, completing the data collection portion of our project. We then created an observational bias simulation thatsimulated our discovery processto placeconstraints onthe underlying number andorbitaldistributionofinnerOortcloudobjects.Thisproducesorder-of-magnitude results for the number of inner Oort cloud objects whose basic parameters are poorly constrained because so few are known.Finally, after identifying the unusual nature of the v distribution, we simulated the inner Oort cloud objectsand the other minor planets with semi-major axes greater than 150 AU and perihelion greater than 30 AU to explore the long-term behaviour of v for different hypothetical planetarymasses in the Solar System. Thesemethods aredescribedin greater detail in the Methods section. Online Content Any additional Methods, Extended Data display items and Source Data are available in the online version of the paper; references unique to these sections appear only in the online paper. Received 11 October 2013; accepted 10 February 2014. 1. Brown, M. E., Trujillo, C. & Rabinowitz, D. Discovery of a candidate inner Oort cloud planetoid. Astrophys. J. 617, 645–649 (2004). 2. Morbidelli, A. & Levison, H. F. Scenarios for the origin of the orbits of the trans- Neptunian objects 2000 CR105 and 2003 VB12 (Sedna). Astron. J. 128, 2564–2576 (2004). 3. Kenyon, S. J. & Bromley, B. C. Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits. Nature 432, 598–602 (2004). 4. Melita, M. D., Larwood, J. D. & Williams, I. P. Sculpting the outer Edgeworth Kuiper belt: stellar encounter followed by planetary perturbations. Icarus 173, 559–573 (2005). 5. Gomes, R. S., Matese, J. J. & Lissauer, J. J. A distant planetary-mass solar companion may have produced distant detached objects. Icarus 184, 589–601 (2006). 6. Levison, H. F. & Morbidelli, A. Models of the collisional damping scenario for ice-giant planets and Kuiper belt formation. Icarus 189, 196–212 (2007). 7. Goldreich, P., Lithwick, Y. & Sari, R. Final stages of planet formation. Astrophys. J. 614, 497–507 (2004). 8. Batygin, K., Brown, M.E.& Betts, H.Instability-driven dynamical evolution modelof a primordially five-planet outer Solar System. Astrophys. J. 744, L3 (2012). 9. Nesvorny´, D. & Morbidelli, A. Statistical study of the early Solar System’s instability with four, five, and six giant planets. Astron. J. 144, 117 (2012). 10. Gomes, R. S., Gallardo, T., Ferna´ndez, J. A. & Brunini, A. On the origin of the high- perihelion scattered disk: the role of the Kozai mechanism and mean motion resonances. Celestial Mech. Dyn. Astron. 91, 109–129 (2005). 11. Soares, J. S. & Gomes, R. S. Comparison of forming mechanisms for Sedna-type objects through an observational simulator. Astron. Astrophys. 553, A110 (2013). 12. Gladman, B. & Chan, C. Production of the extended scattered disk by rogue planets. Astrophys. J. 643, L135–L138 (2006). 13. Ida, S., Larwood, J. & Burkert, A. Evidence for early stellar encounters in the orbital distribution of Edgeworth-Kuiper belt objects. Astrophys. J. 528, 351–356 (2000). 14. Brasser, R. A two-stage formation process for the Oort comet cloud and its implications. Astron. Astrophys. 492, 251–255 (2008). 15. Brasser, R., Duncan, M. J., Levison, H. F., Schwamb, M. E. & Brown, M. E. Reassessing the formation of the inner Oort cloud in an embedded star cluster. Icarus 217, 1–19 (2012). 16. Dukes, D. & Krumholz, M. R. Was the Sun born in a massive cluster? Astrophys. J. 754, 56 (2012). 17. Pfalzner,S.Earlyevolution ofthebirthclusterofthesolarsystem.Astron. Astrophys. 549, A82 (2013). 18. Adams, F. C. The birth environment of the Solar System. Annu. Rev. Astron. Astrophys. 48, 47–85 (2010). 19. Kaib, N. A., Rosˇkar, R. & Quinn, T. Sedna and the Oort cloud around a migrating Sun. Icarus 215, 491–507 (2011). 20. Levison,H.F.,Duncan,M.J., Brasser,R.&Kaufmann,D.E.Captureofthe Sun’sOort cloud from stars in its birth cluster. Science 329, 187–190 (2010). 21. Schwamb, M. E., Brown, M. E., Rabinowitz, D. L. & Ragozzine, D. Properties of the distant Kuiper belt: results from the Palomar Distant Solar System Survey. Astrophys. J. 720, 1691–1707 (2010). 22. Petit, J.-M. et al. The Canada-France Ecliptic Plane Survey—full data release: the orbital structure of the Kuiper belt. Astron. J. 142, 131 (2011). 23. Schwamb,M.E.,Brown,M.E.&Fraser,W.C.ThesmallnumbersoflargeKuiperbelt objects. Astron. J. 147, 2 (2014). 24. Gladman, B. et al. Evidence for an extended scattered disk. Icarus 157, 269–279 (2002). 25. Gomes, R. S., Ferna´ndez, J. A., Gallardo, T. & Brunini, A. in The Solar System Beyond Neptune (eds Barucci, M. A., Boehnhardt, H., Cruikshank, D. P., Morbidelli, A. & Dotson, R.) 259–273 (2008). 26. Becker, A. C. et al. Exploring the outer Solar System with the ESSENCE Supernova Survey. Astrophys. J. 682, L53–L56 (2008). 27. Chen, Y.-T. et al. Discovery of a new member of the inner Oort cloud from the Next Generation Virgo Cluster Survey. Astrophys. J. 775, L8 (2013). 28. Kozai, Y. Secular perturbations of asteroids with high inclination and eccentricity. Astron. J. 67, 591 (1962). 29. Brasser, R., Duncan, M. J. & Levison, H. F. Embedded star clusters and the formation of the Oort cloud. Icarus 184, 59–82 (2006). 30. Sheppard,S.S.et al. A southern skyandgalacticplanesurveyfor brightKuiper belt objects. Astron. J. 142, 98 (2011). Acknowledgements We thank the Dark Energy Camera (DECam) team for obtaining observations during DECam commissioning, D. Norman for scheduling the November 2012 DECam observations, and D. Norman, A. Kunder and K. Holhjem for queue observing in November. T. Abbott and F.Valdeswere very helpful during our December 2012 DECam observations. This project used data obtained with DECam, which was Sedna 2012 VP113 Semimajor axis (AU) Argumentofperihelion(degrees) 200 300 400 500 600100 100 0 –100 Figure 3 | The argument of perihelion for distant objects clusters about 06. All minor planets with perihelion greater than 30 AU as a function of semi- major axis are shown. All bodies with semi-major axis greater than the line at 150AU show a pronounced concentration near v < 0u. Errors on these orbital elements are much smaller than the plotted symbols. This figure appears in histogram form in Extended Data Fig. 1. LETTER RESEARCH 2 7 M A R C H 2 0 1 4 | V O L 5 0 7 | N A T U R E | 4 7 3 Macmillan Publishers Limited. All rights reserved©2014
  • 4. constructed by the Dark Energy Survey collaborating institutions. Observations were in part obtained at the Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation. This paper includes data gathered with the 6.5-m Magellan telescopes located at Las CampanasObservatory,Chile.ThisresearchwasfundedbyNASAPlanetaryAstronomy grant NNX12AG26G and has also been supported by the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., on behalf of the international Gemini partnership of Argentina, Australia, Brazil, Canada, Chile and the USA. Author Contributions C.T. created the moving object detection program for image analysis, developed the discovery statistic simulations, and is the principal investigator of the NASA grant supporting the project. S.S. obtained the telescope time, planned and performed the observations, analysed the data (including the colour measurements) andestimatedtheinnerOortcloudobjectorbitalevolutionusingtheMercuryintegrator. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper. Correspondence and requests for materials should be addressed to C.T. (trujillo@gemini.edu). RESEARCH LETTER 4 7 4 | N A T U R E | V O L 5 0 7 | 2 7 M A R C H 2 0 1 4 Macmillan Publishers Limited. All rights reserved©2014
  • 5. METHODS Discovery survey. In our survey, we imaged 52 square degrees of sky using DECam31 at CTIO, which has the largest field of view (2.7 square degrees) of any 4 morlargertelescope.Thisworkcoversour surveyfromNovemberandDecember 2012, which was comprised of 15 fields with mean ecliptic latitude 212.9u and 4 fields with mean ecliptic latitude 235.5u. We estimate that our survey is sensitive to 95%ofobjectsasfaintasredmagnitudemr 5 24.3witha50%detectionefficiencyat about mr 5 24.5. Solar System objects are found by measuring their apparent motion relative to distant, stationary background stars. Our observational strategy istoobserveeachfieldthreetimeswith 1.5 h to2 h betweeneachimage.Our,3.5-h time base corresponds to about 1.6 arcseconds of motion for a body at 300 AU, which is easily visible in the ,1-arcsecond full-width-at-half-maximum image quality of DECam. The search for new objects in the discovery data is aided by analysis software similar to that used in other surveys32 but adapted specifically to the DECam data set. The analysis software identifies candidate objects in the data set, which are verified by visual inspection of the images. Our search methodology consists of discovery with DECam and additional fol- low-up at the Magellan telescope for any object moving at ,2.5 arcseconds per hour (opposition distance of .50AU or farther). Ninety trans-Neptunians between 30 AU and 50 AU were detected, but only 2012 VP113 was found beyond 50AU and required follow-up. 2012VP113 was discovered on 5 November 2012 UT (Universal time) moving at 1.6arcsec per hour in right ascension. We recovered 2012 VP113 again on 10–11 March 2013 UT, 10–11 August 2013 UT and 28–30 October 2013 UT with IMACS on theMagellan Baade telescope. We further identified 2012 VP113 in archi- val images taken at the Canada–France–Hawaii Telescope on 22 October 2011 UT. The orbital arc33 for 2012VP113 is over two years in duration and is securely known. Colour measurements. The physical surface colours of inner Oort cloud objects can yield information about which formation model is likely. The classical KBOs (,0.2 eccentricity with semi-major axes in the range 40–45 AU) appear to be dom- inated by ultra-red colours, a rare surface colour, probably implying that their surfaces are rich in organic material34 . We therefore obtained the optical colours of 2012VP113 intheSloang,randibandpassesatMagellanon10and11August2013UT. The colours were determined using typical techniques35 . The colour results show that 2012 VP113 is of only moderately red colour (see Extended Data Table 3). A moderate red colour is consistent with formation in the gas giant region, similar to the presumed origin of the outer Oort cloud bodies and scattered disk and not the ultra-reddominatedclassicalKuiperbelt34,35 .Usingourphotometry,the83-AU helio- centric distance of 2012VP113, and assuming a moderate albedo of 15% (ref. 36), we determine that 2012VP113 is 450km in diameter. Observational bias simulation. Because minor planets are visible in reflected sunlight, there is a 4th-power drop in observed flux with increasing heliocentric distance, avery strongbias against findingdistant objects. Tomodelandultimately correct for these biases, we constructed a simple model of our observational tech- nique. Underlying parameters characteristic of the inner Oort cloud object orbits are assumed, and then the position and brightness of each object at a particular time is calculated. The model population is based on a range of size distributions22 and a Gaussian scattered KBO inclination distribution37 with a uniform eccent- ricity distribution and a power-law semi-major axis distribution32 . The values of the assumed parameters are detailed in Extended Data Table 1, and are used to createthe ensemble ofsimulatedobjects.We notethat althoughwe believe we have chosen a reasonable range of parameters for the size and orbital distribution of the innerOort cloudobjects, thesearebasedentirelyonextrapolations fromthe KBOs. Revisionsare likelyinthefutureifthesizeandorbitaldistributionsoftheinnerOort cloud objects deviate significantly from that of the KBOs. Once the model popu- lation is generated, any objects brighter than our survey sensitivity limit mr 5 24.3 and withinour distancesensitivity(from50 AU, our minimumheliocentricdistance follow-up criterion, to beyond 300 AU, where we begin to be limited by slow object motion) are tallied as a function of ecliptic latitude. Given the typical ecliptic latitudes of our observed fields, this tally yields the number and orbital character- istics of the inner Oort cloud objects we should have found if our assumed distri- bution matches the real distribution. In our observational model, we have included our surveyas well as the deepest near all-sky survey to date, which detected Sedna21 , because it is an additional constraint on the inner Oort cloud objects. Our first goal of the bias simulation is to examine the perihelion distribution of both Sedna and 2012 VP113 because both objects were found with large perihelia (.75 AU) in surveys that were sensitive to objects with smaller perihelia. Figure 2 and Extended Data Table 1 describe an assumed size distribution of q9 5 5 (similar to the bright KBOs and the large bodies of other stable populations)22,23,38 ; how- ever, we also test size distributions with q9 5 3.5, 4.0 and 4.5, which are found for smaller objects in some small-body populations39 . We find that the counting statistics for objects in the 50–75 AU region are dependent on the size distribution assumed. For the size distributions of q9 5 3.5, 4.0, 4.5 and 5.0, we find the prob- ability of finding Sedna and 2012 VP113 with perihelion beyond 75 AU when the underlying size distribution extends down to 50 AU is 17%, 7%, 5% and 1%, respec- tively. Given that we have no knowledge of the underlying size distribution of the inner Oort cloud objects, we cannot statisticallyrule out the possibility that the 50– 75 AU regioncouldbepopulatedwithinnerOortcloudobjects.However,itappears to have a paucity of inner Oort cloud objects compared to q . 75 AU, especially if the inner Oort cloud objects have a steep size distribution like that of the bright KBOs. Our second goal for the observational simulation is to assess whether the clus- tering of v near 0u found for Sedna, 2012 VP113 and the objects listed in Extended Data Table 2 could be attributed to a bias effect. An v of 0u or 180u means that the perihelion of the orbit is within the ecliptic plane. Most eccentric bodies are found near perihelion where they are brightest, so this would suggest that surveys restricted to the ecliptic plane should have a bias for finding objects with v around 0u or 180u. Wecanreject this bias as an explanation ofthe observed v trend fortwo reasons. First,anybias for v 5 0u (meaningthatatperiheliontheobject crosses the ecliptic moving southward) is the same as the bias for v 5180u (meaning that at perihelion the object crosses the ecliptic moving northward) because observational discovery surveys have no bias against direction of motion at this level. Because no objects were found with v < 180u and all had v < 0u, the v clustering is already in- consistent with any observational effect. Second, the surveys that found the objects with clustered v were often off-ecliptic or all-sky surveys, which would not have a bias for v 50u or 180u. Extended Data Table 2 shows the discovery ecliptic latitude b of the objects. In our observational simulations, we find that the v 50u or 180u discovery bias is only significant when discovery surveys probe jbj , 10u, which is true for only about half of the objects listed. Even when this bias is present, it should affect v 5 0u and 180u equally, for the reasons discussed above. Thus, we know of no observational bias which could explain why all objects with q . 30AU and a .150 AU would have v < 340u 655u and none have 90u , v , 270u. Dynamical simulation. We used the Mercury integrator40 to simulate the long- term behaviour of v for the inner Oort cloud objects and objects with semi-major axes greater than 150AU andperihelia greater than Neptune. The goal of this simu- lationwas to attempt to explain the v clustering. The simulation shows that for the currently known mass in the Solar System, v for all objects circulates on short and differing timescales dependent on the semi-major axis and perihelion (for example, 1,300 Myr, 500 Myr, 100 Myr and 650 Myr for Sedna, 2012 VP113, 2000 CR105 and 2010 GB17, respectively). We illustrate an example of the v circulation behaviour in Extended Data Fig. 2. Given that the inner Oort cloud objects and the objects listed in Extended Data Table 2 show a preference for v near 0u but not 180u, and that the timescale for v circulation is short (and differs between objects) compared to the ageoftheSolarSystem,wesuggestthattheperturberconstrainingvfortheseobjects is still present and as yet unidentified. To test the massive perturber hypothesis as a source in constraining v, we ran several simulations with a single body of 2–15 Earth masses in a circular low in- clinationorbitbetween 200 AU and300AU usingtheMercury integrator. Ingeneral it was found that the v for the inner Oort cloud objects 2012 VP113 and Sedna routinely librated with an amplitude of about 660u around v 5 0u for billions of years. The librating behaviour of v for 2012 VP113 with an additional body of 5 Earth masses at 210AU is illustrated in Extended Data Fig. 3, where 2012 VP113 spends only 3% of its time in the 90u , v , 270u region. Sedna spent 1% of its time in this region. For the low-i super-Earth simulations, the lower-perihelion objects did not librate for long periods as did 2012 VP113 and Sedna and spent 30% to 50% of their time with 90u , v , 270u. We also ran simulations with highly inclined Neptune mass bodies at about 1,500 AU and found most objects exhibiting short v librations, which suggests some interesting potential future simulation prospects. Very few dynamical simulations of inner Oort cloud object formation have con- sidered the distribution of the argument of perihelion for inner Oort cloud objects, and so far none have produced a population that is asymmetric when comparing v50utov5180u.Possiblestrongstochasticeventsmightcreateasymmetricpopu- lations, but this needs to be confirmed through future simulations. We note that the planetconfigurationswetestedarenotuniqueandtherearelikelytobemanypossible orbital patterns for an unseen perturber that could produce a v constraint among the most distant bodies in the Solar System41,42 . 31. Diehla,T.TheDarkEnergySurveyCamera(DECam).Phys.Proc.(Proc.2ndInt.Conf. on Technology and Instrumentation in Particle Physics, TIPP 2011) 37, 1332–1340 (2012). 32. Trujillo, C. A., Jewitt, D. C. & Luu, J. X. Properties of the trans-Neptunian belt: statistics from the Canada-France-Hawaii telescope survey. Astron. J. 122, 457–473 (2001). 33. Bernstein,G.&Khushalani,B.OrbitfittinganduncertaintiesforKuiperbeltobjects. Astron. J. 120, 3323–3332 (2000). 34. Doressoundiram, A., Boehnhardt, H., Tegler, S. C. & Trujillo, C. in The Solar System Beyond Neptune (eds Barucci, M. A., Boehnhardt, H., Cruikshank, D. P., Morbidelli, A. & Dotson, R.) 91–104 (2008). LETTER RESEARCH Macmillan Publishers Limited. All rights reserved©2014
  • 6. 35. Sheppard, S. S. The colors of extreme outer Solar System objects. Astron. J. 139, 1394–1405 (2010). 36. Stansberry, J. et al. in The Solar System Beyond Neptune (eds Barucci, M. A., Boehnhardt, H., Cruikshank, D. P., Morbidelli, A. & Dotson, R.) 161–179 (2008). 37. Gulbis, A.A. S. etal.Unbiasedinclination distributions for objectsintheKuiperbelt. Astron. J. 140, 350–369 (2010). 38. Sheppard, S. S. & Trujillo, C. A. The size distribution of the Neptune Trojans and the missing intermediate-sized planetesimals. Astrophys. J. 723, L233–L237 (2010). 39. Petit, J.-M., Kavelaars, J. J., Gladman, B. & Loredo, T. in The Solar System Beyond Neptune (eds Barucci, M. A., Boehnhardt, H., Cruikshank, D. P., Morbidelli, A. & Dotson, R.) 71–87 (2008). 40. Chambers, J. E. Mercury: a Software Package for Orbital Dynamics (Astrophysics Source Code Library, 2012). 41. Lykawka, P. S. & Mukai, T. An outer planet beyond Pluto and the origin of the trans- Neptunian belt architecture. Astron. J. 135, 1161–1200 (2008). 42. Lykawka, P. S. & Ito, T. Terrestrial planet formation during the migration and resonance crossings of the giant planets. Astrophys. J. 773, 65 (2013). RESEARCH LETTER Macmillan Publishers Limited. All rights reserved©2014
  • 7. Extended Data Figure 1 | Histogram of v for minor planets with q . 30 AU. This is similar to Fig. 3 but in histogram form. The bodies with a . 150 AU are shown as a black line (multiplied by a factor of ten for clarity) and bodies with a , 150 AU are shown as a dotted line. The two distributions differ according to Kuiper’s test with a significance of 99.9%. LETTER RESEARCH Macmillan Publishers Limited. All rights reserved©2014
  • 8. Extended Data Figure 2 | The v cycling of 2012 VP113 in the current Solar System. We note that over the course of 500Myr, the argument of perihelion v moves uniformly across all values. All inner Oort cloud Objects (Table 1) and other distant objects (Extended Data Table 2) are expected to exhibit this behaviour on differing timescales, so the observation that all are restricted to v near 0u is inconsistent with the current dynamical environment in the Solar System. Because these are simulated plots, there are no error bars associated with data points. RESEARCH LETTER Macmillan Publishers Limited. All rights reserved©2014
  • 9. Extended Data Figure 3 | The libration of v for 2012 VP113 with an assumed object five times the mass of Earth at 210 AU. 2012 VP113 librates about v 5 0u for most of theduration of theSolar System. This behaviour could explain why the two inner Oort cloud Objects (Table 1) and all objects with semi-major axes greater than 150AU and perihelia greater than Neptune’s (Extended Data Table 2) have v < 0u. The choice of mass and orbit of the perturber is not unique. Many possible distant planetary bodies can produce the pictured Kozai resonance behaviour, but the currently known Solar System bodies cannot. These are simulated plots, so there are no error bars associated with data points. LETTER RESEARCH Macmillan Publishers Limited. All rights reserved©2014
  • 10. Extended Data Table 1 | Model parameters for the q9 5 5 inner Oort Cloud observational bias and population study Size distributions of q9 5 3.5, 4.0 and 4.5 were also studied, with all parameters identical except that N 5 4 3 106 , N 5 1 3 107 and N 5 4 3 107 , respectively. The population mass varies greatly with the model assumed, M 5 5, 1/30, 1/40 and 1/80 Earth masses for q9 5 3.5, 4.0, 4.5 and 5.0. RESEARCH LETTER Macmillan Publishers Limited. All rights reserved©2014
  • 11. Extended Data Table 2 | Orbital elements of extreme Solar System bodies This includes all bodies with perihelia greater than 30 AU and semi-major axes greater than 150 AU, except for the two inner Oort cloud objects listed in Table 1. The JPL Horizons heliocentric orbital elements are given as semi-major axis (a), eccentricity (e), inclination (i), longitude of ascending node (V), argument of perihelion (v), and perihelion distance (q), which has been derived from a and e. Time of perihelion (tp) and epoch are specified in terms of Julian Date (JD). The precision to which quantities are known is reflected in the number of significant digits. As discussed in the text, the first three objects may have origins similar to those of the inner Oort cloud objects, although their perihelia are far lower than that of either Sedna or 2012 VP113. We also show the discovery ecliptic latitude b of the objects because the bias for finding bodies with v 5 0u or 180u is relatively low when |b | . 10u (Methods). LETTER RESEARCH Macmillan Publishers Limited. All rights reserved©2014
  • 12. Extended Data Table 3 | Colours of 2012 VP113 These colours yield a spectral slope of 13 6 2% per 100 nm and a reduced magnitude in the R-band of 3.8 6 0.04. The reduced magnitude suggests that 2012 VP113 is about 450 km in diameter, assuming a moderate albedo of 15%. RESEARCH LETTER Macmillan Publishers Limited. All rights reserved©2014